This invention relates to methods of burning refuse or garbage and has particular reference to boilers for burning refuse as part of the fuel.
The steady increase in the volume of urban waste is causing increasing problems associated with its disposal. Waste is becoming steadily less dense which means that the volume is increasing disproportionately with the weight of the material. The calorific value of refuse has steadily increased through the past 30 years because the amount of paper and plastics material in refuse has increased while the ash content has decreased.
As the number of suitable dumping sites reduces, particularly in or near large urban areas, there has been an increase in the amount of refuse which is burnt in refuse incinerators to reduce the volume to more manageable levels. A certain amount of heat has been recovered from these incinerators but the reduction in volume has hitherto been more important than the heat recovery. To date, there have been very few successful installations which use refuse as a significant source of energy.
An investigation of the literature and prior patent specifications has revealed two British Pat. Nos. 366,307 and 436,708, which were published in 1932 and 1935 respectively. It is not known whether the inventions described in these patent specifications were ever used in practice but it can be seen that the invention described in both of the specifications would have been difficult to use in practice. Both involved extensive modifications to existing furnaces and both involved the addition of large quantities of rubbish to an existing bed of coal. In both cases, the rubbish was added from the front end of the furnaces, ie from the front wall, and this means that it was not possible to obtain suspension firing of the rubbish. This would result in a thick layer of rubbish being applied to the bed of coal and the advantages of the present invention would not have been obtained. Even if the rubbish were to have been injected rapidly from the front wall, it would have fallen to the back of the furnace onto the bed of coal and would not have burned efficiently before passing to the ash disposal unit.
In British Pat. No. 366,307, the moving bed of coal is agitated by reciprocating fire bars to agitate the coal bed and to intermix the refuse and coal. This would be a difficult system to install and could result in large areas of refuse in the coal being burnt through and offering an easy escape route for air in the air chamber beneath the fire grate.
With the arrangement described in British Pat. No. 436,708, a very thick layer of refuse is applied to a relatively thin layer of coal, again at the front of the chain grate and it is known that these thick beds of refuse burn incompletely and are unsatisfactory from the point of view of generating steam.